The International Consortium in Psychiatric Epidemiology (ICPE) was established in 1998 by WHO to carry out cross-national comparative studies of the prevalences and correlates of mental disorders. This article describes the findings of ICPE surveys in seven countries in North America (Canada and USA), Latin America (Brazil and Mexico), and Europe (Germany, Netherlands, and Turkey), using a version of the WHO Composite International Diagnostic Interview (CIDI) to generate diagnoses. The results are reported using DSM-III-R and DSM-IV criteria without diagnostic hierarchy rules for mental disorders and with hierarchy rules for substance-use disorders. Prevalence estimates varied widely--from > 40% lifetime prevalence of any mental disorder in Netherlands and the USA to levels of 12% in Turkey and 20% in Mexico. Comparisons of lifetime versus recent prevalence estimates show that mental disorders were often chronic, although chronicity was consistently higher for anxiety disorders than for mood or substance-use disorders. Retrospective reports suggest that mental disorders typically had early ages of onset, with estimated medians of 15 years for anxiety disorders, 26 years for mood disorders, and 21 years for substance-use disorders. All three classes of disorder were positively related to a number of socioeconomic measures of disadvantage (such as low income and education, unemployed, unmarried). Analysis of retrospective age-of-onset reports suggest that lifetime prevalences had increased in recent cohorts, but the increase was less for anxiety disorders than for mood or substance-use disorders. Delays in seeking professional treatment were widespread, especially among early-onset cases, and only a minority of people with prevailing disorders received any treatment. Mental disorders are among the most burdensome of all classes of disease because of their high prevalence and chronicity, early age of onset, and resulting serious impairment. There is a need for demonstration projects of early outreach and intervention programmes for people with early-onset mental disorders, as well as quality assurance programmes to look into the widespread problem of inadequate treatment.